Low-level detection of threat agents and illicit substances (e.g., explosive, narcotics and organophosphorus compounds (OPCs)) remains a long-standing goal of modern instrument platforms. Analytical techniques for direct vapor detection of threat agents and illicit substances remain limited because significant improvements in sensitivity must yet be attained if automated technologies are to be of practical use. For example, equilibrium vapor pressure (saturated) of RDX explosive at 25° C. provide a concentration of ˜5 parts-per-trillion (ppt). Because real-world analyses must achieve detection below saturation levels, sensitivity must be substantially better than this 5 ppt threshold. To complicate matters, improvements in sensitivity without improvements in selectivity are counterproductive, as increasing sensitivity effectively raises the chemical noise, which offsets improvements to upstream components. Also, the ability to detect multiple analytes from a single sampling event is limited by the fact that detection methods used for certain analytes do not always work to detect other types of analytes. As such, multiple sampling events and/or instruments must be utilized to detect various threat agents and/or illicit substances. Accordingly, new methods and systems are needed to provide accurate detection of such analytes, particularly at ultra-low levels, and without requiring multiple sampling events and/or analyses.